Cure control of silicone rubber compositions

Abstract: An addition curing type silicone rubber composition comprising an aliphatic unsaturated group - containing organopolysiloxane, an organohydrogenpolysiloxane and a platinum series catalyst is further blended with a triazine or sulfide control agent to provide a composition which is stable and resistant against premature gelation at room temperature and thus has long-term storage stability. Heating causes the composition to be quickly vulcanized at an acceptable rate into a silicone rubber having satisfactory physical properties.

Description: This disclosure relates to addition-curing type silicone rubber compositions using a platinum series catalyst, and more particularly, to silicone rubber compositions which are stabilised against premature gelation. Prior Art Conventional silicone rubber compositions are loaded with platinum series catalysts for promoting addition reaction between an aliphatic unsaturated group-containing organopolysiloxane and an organohydrogenpolysiloxane. The crosslinking rate of this addition reaction is generally high albeit some dependency on the activity of the catalysts. As a consequence, control agents or curing inhibitors are added to most compositions for preventing premature gelation at room temperature, thereby imparting potential curability to the compositions in the sense that the compositions are stable and remain unchanged over a long period of time, but can be cured by heating. Known such control agents are ethylenically unsaturated isocyanurates and compounds having a hydroperoxide -C-O-O-H structure as disclosed in Japanese Patent Publication (JP-B) Nos. 41626/1980 and 20340/1982. However, the former control agents are less effective for controlling the platinum series catalysts. The latter control agents must be added in a carefully controlled amount because otherwise vulcanization does not take place or the vulcanization rate is slow. For preventing premature gelation at room temperature, it is also proposed to contain the platinum series catalyst and the organohydrogenpolysiloxane in separate packs which are mixed on use. This requires an additional operation of mixing two components. Although a number of premature gelation inhibitors are currently available, many control agents, typically acetylene alcohol agents are volatile. Since they lose control ability by volatilization, they cannot survive over a long working life. Such control agents must be added in excess of the necessity. A general aim addressed herein is the provision of new addition-curing silicone rubber compositions having cure control means for stabilising against premature gelation. Proposed and particular aims include (independently) the ability to provide the composition in one-pack form, long working life, and good workability. Another general aim is to propose new and useful uses of compounds as cure control agents in such a context. A first aspect of the invention is a silicone rubber composition comprising (A) organopolysiloxane of the following average compositional formula (1): R 1 n SiO (4 - n)/2 wherein groups R 1 , which may be identical or different, are selected from substituted and unsubstituted monovalent hydrocarbon groups and include at least two aliphatic unsaturated groups per molecule, and n is in the range 1.98 to 2.02; (B) organohydrogenpolysiloxane of the following average compositional formula (2) : R 2 a H b SiO (4 - a - b)/2 having per molecule at least two hydrogen atoms attached to silicon atom, and wherein groups R 2 , which may be identical or different, are selected from substituted and unsubstituted monovalent hydrocarbon groups and a and b satisfy the relationships 0 ≤ a < 3, 0.005 ≤ b ≤ 2 and 0.8 ≤ a + b ≤ 3.0; (C) a platinum series catalyst, and (D) cure control agent selected from sulphides of the following general formula (4) : R 6 -(S) m -R 7 wherein each of R 6 and R 7 is a monovalent hydrocarbon group having at least 3 carbon atoms, or a monovalent hydrocarbon group including an ester group, or R 6 and R 7 together form a ring structure, and m is from 1 to 3. Preferably the composition is a one-pack composition. We have found that by blending the specified organosulphides in the silicone rubber composition, premature gelation can be prevented to ensure a long working life at room temperature without detracting from the physical properties of the rubber. A further aspect (as set out in claim 10) is the use of the specified sulphides as curing control agent to inhibit room-temperature gelation in relation to the addition-curing reaction of a one-pack composition of the components (A) to (C) as specified above. A first essential component (component (A)) of the silicone rubber composition is organopolysiloxane of the following average compositional formula (1). R 1 n SiO (4 - n)/2 R 1 are substituted or unsubstituted monovalent hydrocarbon groups. Letter n is a positive number of 1.98 to 2.02. In formula (1), R 1 is a substituted or unsubstituted monovalent hydrocarbon group preferably having 1 to 20 carbon atoms, more preferably 1 to 8 carbon atoms. The R 1 groups include aliphatic saturated hydrocarbon groups, for example, alkyl groups such as methyl, ethyl, propyl and butyl and cycloalkyl groups such as cyclohexyl; aliphatic unsaturated hydrocarbon groups, for example, alkenyl groups such as vinyl, allyl, butenyl and hexenyl groups; aryl groups such as phenyl and tolyl groups; and substituted such groups. The skilled person is familiar with such substitution options and readily able to select suitable substituted groups e.g. wherein some or all of the hydrogen atoms attached to carbon atoms are replaced by halogen atoms or cyano groups, for example, chloromethyl, trifluoropropyl and cyanoethyl groups. Generally preferred are methyl, phenyl and trifluoropropyl groups. Although the R 1 groups may be identical or different, the organopolysiloxane should contain at least two aliphatic unsaturated groups in a molecule, preferably at least two alkenyl groups having 2 to 10 carbon atoms, especially at least two vinyl groups. The aliphatic unsaturated group should preferably be contained in an amount of 0.001 to 20 mol%, more preferably 0.025 to 5 mol% of the entire R 1 groups. Letter n is a positive number of 1.98 to 2.02. The organopolysiloxane of formula (1) is generally preferred to be linear although it is acceptable to use different molecular structures, for example, branched or three-dimensional network organopolysiloxanes. The organopolysiloxane preferably has a degree of polymerization of 100 to 10,000, more preferably 5,000 to 8,000. A second essential component (component (B)) is organohydrogenpolysiloxane which serves as a curing agent, and has the average compositional formula (2). R 2 a H b SiO (4 - a - b)/2 R 2 are substituted or unsubstituted monovalent hydrocarbon groups. Letter a satisfies 0 ≦ a < 3, b is a positive number of from 0.005 to 2, and the sum of a and b is from 0.8 to 3.0. The organohydrogenpolysiloxane should have at least two hydrogen atoms each attached to a silicon atom in a molecule. The groups represented by R 2 may be identical or different and the options are the same as described for R 1 in formula (1). The organohydrogenpolysiloxane may be linear, branched or cyclic and preferably has a degree of polymerization of up to 300. Examples of the organohydrogenpolysiloxane include a diorganopolysiloxane end-blocked with a dimethylhydrogensilyl group, a copolymer of a dimethylsiloxane unit, a methylhydrogensiloxane unit, and a terminal trimethylsiloxy unit, a low viscosity fluid consisting of a dimethylhydrogensiloxane unit (H(CH 3 ) 2 SiO 1/2 unit) and a SiO 2 unit, 1,3,5,7-tetrahydrogen-1,3,5,7-tetramethylcyclotetrasiloxane, 1-propyl-3,5,7-trihydrogen-1,3,5,7-tetramethylcyclotetrasiloxane, and 1,5-dihydrogen-3,7-dihexyl-1,3,5,7-tetramethylcyclotetrasiloxane. Desirably the organohydrogenpolysiloxane is blended in such amounts that the hydrogen atom directly attached to a silicon atom in the organohydrogenpolysiloxane is available in an amount of 50 to 500 mol% based on the aliphatic unsaturated group (e.g., alkenyl group) in the organopolysiloxane or component (A). A third essential component (component (C)) is a platinum series catalyst which may be selected from conventional known ones. Exemplary platinum catalysts include elemental platinum alone, platinum compounds, platinum compositions, chloroplatinic acid, and complexes of chloroplatinic acid with alcohols, aldehydes, ethers, and olefins. The platinum series catalyst is used in a catalytic amount, preferably such that platinum atom is available in an amount of 1 to 2,000 ppm based on the organopolysiloxane or component (A). According to the present invention a sulphide of general formula (4) is blended as a fourth essential component (component (D)). R 6 - (S) m - R 7 Each of R 6 and R 7 is a monovalent hydrocarbon group having at least 3 carbon atoms or a monovalent hydrocarbon group having an ester bond. Alternatively R 6 and R 7 , taken together, form a ring structure. Letter m is a positive number of 1 to 3. The sulphide compound functions as a control agent for a silicone rubber composition. When added in appropriate amounts, we find it effective for improving the storage stability and hence, the pot life of a silicone rubber composition without the aid of another well-known control agent while preventing premature gelation at room temperature and avoiding any lowering in rate of subsequent vulcanization. When the silicone rubber composition including such a sulphide is heated at normal vulcanization temperatures of about 100 to 130°C or higher, we find a vulcanization rate equal to or slightly higher than the conventional rate. That is, the sulphide serves as a vulcanizing agent having temperature dependency. In formula (4), each of R 6 and R 7 is a monovalent hydrocarbon group having at least 3 carbon atoms, desirably 3 to 50 carbon atoms, more desirably 7 to 50 carbon atoms or (such) a monovalent hydrocarbon group having an ester linkage in the molecular backbone. Alternatively R 6 and R 7 , taken together, form a ring structure having at least 3 carbon atoms in total, desirably 3 to 50 carbon atoms, more desirably 7 to 50 carbon atoms, for example, a cycloalkyl group. Example of the monovalent hydrocarbon group include alkyl groups, aryl groups, aralkyl groups, and alkenyl groups. Those sulphides of formula (4) wherein either one or both of R 6 and R 7 are monovalent hydrocarbon groups having less than 3 carbon atoms have an increased tendency to poison the platinum catalyst as a sulphur compound, which results in a composition not being crosslinked or requires to increase the effective platinum amount. Those sulphides of formula (4) wherein either one or both of R 6 and R 7 are monovalent hydrocarbon groups having more than 50 carbon atoms would sometimes be less effective for preventing premature gelation and a lowering of vulcanization rate. Letter m is a positive number of 1 to 3, preferably 1 or 2. That is, the sulphide is preferably a monosulphide or disulphide, with the monosulphide being most preferred. With m of 4 or higher, the compound has an increased catalyst poisoning tendency, failing to achieve the intended advantages or resulting in a composition not being crosslinked. Several illustrative, non - limiting examples of the sulphide are given below. S(-C 12 H 23 ) 2 , S 2 (-C 10 H 19 ) 2 , S(-CH 2 -Ph) 2 , H 11 C 5 - S - C 12 H 23 , Ph - C 2 H 4 - S - S - C 10 H 21 , Note that Ph is phenyl. Preferably the sulphide is used in amounts of 0.005 to 5 parts, more preferably 0.01 to 1 part by weight per 100 parts by weight of the organopolysiloxane component (A). Less than 0.005 part of the sulphide would be less effective for preventing premature gelation whereas more than 5 parts of the sulphide would detract from rubber physical properties. Satisfactory results are obtained when the formula (4) sulphide is used alone although it may be combined with another control agent if desired. When it is combined with another control agent, a significantly enhanced control effect is expected. For example, when the sulphide is combined with a sublimable control agent such as an acetylene alcohol, the silicone rubber composition can be improved in long-term working at room temperature. For such combined use, the other control agent may be selected from well-known ones, for example, acetylene alcohols, vinylsiloxanes having a low degree of polymerization (3 to 20, especially 3 to 10) such as 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, hydroperoxides, and ethylenically unsaturated isocyanurates. Moreover, there can be used those control agents which are otherwise rejected for addition alone because of their inadequate control effect, for example, peroxides containing a carbon-to-carbon unsaturated bond such as 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3. The amount of the other control agent is desirably selected in a range giving effective results as explained herein, preferably 0.001 to 1 part by weight per 100 parts by weight of the organopolysiloxane or component (A). Preferably reinforcing silica powder is blended in the silicone rubber composition because a silicone rubber having improved mechanical strength can be produced. For mechanical strength improvement, the reinforcing silica filler should preferably have a specific surface area of at least 50 m 2 /g, especially 100 to 300 m 2 /g as measured by a BET method. Examples include fumed silica and precipitated silica, which may have been treated at the surface to be hydrophobic with suitable agents such as hexamethyldisilazane and chlorosilanes. Preferably about 5 to 70 parts by weight, more preferably about 10 to 50 parts by weight of silica filler is blended with 100 parts by weight of the organopolysiloxane or component (A). Less than 5 parts of the silica powder would be too small to achieve reinforcement whereas more than 70 parts of the silica powder may rather reduce workability and mechanical strength. Any optional components may be added to the inventive silicone rubber composition. Such optional components include extending fillers such as ground quartz, calcium carbonate, and carbon; and organic and inorganic blowing agents for forming sponge products, such as azobisisobutyronitrile, dinitropentamethylenetetramine, and benzenesulfonhydrazide. The blowing agent is preferably added in an amount of 3 to 10 parts by weight to 100 parts by weight of the silicone rubber composition. With such a blowing agent added, a spongy silicone rubber can be obtained. It will be understood that any of additives commonly blended in conventional silicone rubbers may be added to the inventive silicone rubber composition (insofar as the effects of our proposals are not negated). Such additives include coloring agents, heat resistance enhancers, mold release agents, and filler dispersing agents (e.g., diphenylsilane diol, alkoxysilanes, carbon functional silane, and silanol group-containing low molecular weight siloxanes). They may be added in conventional amounts although the amount of the filler dispersing agent added should desirably be minimized so as not to impair the advantages of our proposals. The silicone rubber composition may be prepared by mixing the essential and optional components mentioned above. Preferably the platinum series catalyst is first mixed with the organosiloxane gum and the aliphatic unsaturated group-containing organopolysiloxane having a low degree of polymerization to form a paste, which is mixed with the remaining components. Owing to the addition of the sulphide component (D), the silicone rubber composition is, we find, stable at room temperature and does not readily gel although it contains an aliphatic unsaturated group-containing organopolysiloxane, an organohydrogenpolysiloxane and a platinum series catalyst in a single pack. When it is desired to cure the composition, the composition may be heated to 100 to 500°C for several seconds to 5 hours. Then using any of conventional vulcanization techniques including atmospheric pressure hot air vulcanization, steam vulcanization and compression molding, the composition can be readily cured into a silicone rubber having satisfactory physical properties, which will find similar uses to conventional silicone rubbers. There has been described an addition curing type silicone rubber composition comprising an aliphatic unsaturated group-containing organopolysiloxane, an organohydrogenpolysiloxane and a platinum series catalyst wherein a sulphide is blended to provide a composition stable and resistant against premature gelation at room temperature and with consequently improved long-term storage stability. In addition, simple heating causes the composition to be quickly vulcanized at an acceptable vulcanization rate into a silicone rubber having satisfactory physical properties. The composition has a significantly extended working life at room temperature. The resulting silicone rubber can find a wide variety of uses as do conventional silicone rubbers. EXAMPLE Examples of the present invention are given below by way of illustration and not by way of limitation. All parts are by weight. Examples 1 to 3 A platinum paste B was prepared by mixing 10 grams of a low-degree-of-polymerization dimethylvinylpolysiloxane substituted solution of a chloroplatinic acid-alcohol solution (having a vinyl content 0.7 mol% and a platinum content of 1.0%) and 90 grams of an organopolysiloxane having a vinyl content of 0.15 mol% and an average degree of polymerization of 5,000. To 100 parts of an organopolysiloxane consisting of 99.85 mol% of a dimethylsiloxane (CH 3 ) 2 SiO unit and 0.15 mol% of a methylvinylsiloxane (CH 3 )(CH=CH 2 )SiO unit and terminated with a dimethylvinylsiloxane (CH 3 ) 2 (CH=CH 2 )SiO 1/2 unit at either end were added 45 parts of fumed silica (Aerosil 200 commercially available from Nippon Aerosil K.K.) and 7 parts of hydroxyl-end-blocked dimethylsilicone fluid having a degree of polymerization (n) of 10. The ingredients were mixed in a kneader and then heat treated at 160 to 170°C for 2 hours, obtaining a rubber compound. To 100 parts of the rubber compound were added 0.5 part of platinum paste B as a curing agent, 0.05 or 0.1 part of di-n-hexylsulphide, and 1.2 parts of organohydrogenpolysiloxane having a Si-H content of 0.005 mol/g. The ingredients were milled to form silicone rubber compositions (Examples 1 and 2). A silicone rubber composition (Example 3) was prepared by the same procedure as Example 1 expect that 0.1 part of a conventional known control agent, ethynylcyclohexanol, was further added to the composition of Example 1. Comparative Examples 1 and 2 A silicone rubber composition (Comparative Example 1) was prepared by the same procedure as Example 1 except that di-n-hexylsulphide was omitted. A silicone rubber composition (Comparative Example 2) was prepared by the same procedure as Example 1 except that di-n-hexylsulphide was omitted and ethynylcyclohexanol was added instead. Examples 4 to 6 A platinum paste C was prepared by mixing 10 grams of a low-degree-of-polymerization dimethylvinylpolysiloxane substituted solution of a chloroplatinic acid-alcohol solution (having a vinyl content 0.7 mol% and a platinum content of 1.0%), 1.0 grams of 2,4,6-tris(t-butylperoxy)-1,3,5-triazine (commercially available from Kayaku Akuzo K.K.), and 89 grams of an organopolysiloxane having a vinyl content of 0.15 mol% and an average degree of polymerization of 5,000. To 100 parts of the same rubber compound as in Example 1 were added 0.5 part of platinum paste C as a curing agent, 0.02 part of ditridecyl-3,3'-thiodipropionate, and 1.2 parts of organohydrogenpolysiloxane having a Si-H content of 0.005 mol/g. The ingredients were milled to form a silicone rubber composition (Example 4). Silicone rubber compositions (Examples 5 and 6) were prepared by the same procedure as Example 4 except that 0.01 part of di-n-hexylsulphide or n-octylsulphide was added instead of ditridecyl-3,3'-thiodipropionate. Each of the resultant silicone rubber compositions was allowed to stand in a dryer at 40°C such that no air was directly blown to the composition. A gel time was measured. The gelling point was judged by milling a sheet lmm thick between two rolls ten rounds, wrapping the sheet around roll, and observing whether the sheet surface texture was smooth. When the surface texture was smooth, the sheet was judged not gelled. The results are shown in Tables 1 and 2. Composition (pbw) Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Rubber compound 100 100 100 100 100 Platinum paste B 0.5 0.5 0.5 0.5 0.5 Di-n-hexylsulphide 0.05 0.1 0.05 - - Ethynylcyclohexanol - - 0.1 - 0.1 Organohydrogenpolysiloxane 1.2 1.2 1.2 1.2 1.2 Gel time at 40°C 6 days 15 days 10 days 15 minutes 1 day Composition (pbw) Example 4 Example 5 Example 6 Rubber Composition 100 100 100 Platinum paste C 0.5 0.5 0.5 Ditridecyl-3,3'-thiodipropionate 0.02 - - Di-n-decyldisulphide - 0.01 - n-Octylsulphide - - 0.01 Organohydrogenpolysiloxane 1.2 1.2 1.2 Gel time at 40°C 30 days 28 days 30 days As is evident from Tables 1 and 2, silicone rubber compositions exploiting our new proposals have a very long gel time at 40°C and are potentially curable compositions which are stabilized against premature gelation. While this invention has been described with respect to specific examples, it will be understood by those skilled in the art that various changes and modifications may be made to the examples in the light of the general teachings herein.

Claims: A silicone rubber composition comprising (A) organopolysiloxane of the following average compositional formula (1): R 1 n SiO (4 - n)/2 wherein groups R 1 , which may be identical or different, are selected from substituted and unsubstituted monovalent hydrocarbon groups and include at least two aliphatic unsaturated groups per molecule, and n is in the range 1.98 to 2.02; (B) organohydrogenpolysiloxane of the following average compositional formula (2) : R 2 a H b SiO (4 - a - b)/2 having per molecule at least two hydrogen atoms attached to silicon atom, and wherein groups R 2 , which may be identical or different, are selected from substituted and unsubstituted monovalent hydrocarbon groups and a and b satisfy the relationships 0 ≤ a < 3, 0.005 ≤ b ≤ 2 and 0.8 ≤ a + b ≤ 3.0; (C) a platinum series catalyst, and (D) cure control agent selected from sulphides of the following general formula (4): R 6 -(S) m -R 7 wherein each of R 6 and R 7 is a monovalent hydrocarbon group having at least 3 carbon atoms, or a monovalent hydrocarbon group including an ester group, or R 6 and R 7 together form a ring structure, and m is from 1 to 3. A composition according to claim 1 in which in the sulphide cure control agent (D) each of R 6 and R 7 is alkyl, aryl, aralkyl or alkenyl having from 3 to 50 carbon atoms, optionally with an ester linkage in a hydrocarbon molecular backbone thereof, or in which R 6 and R 7 together form a cycloalkyl group. A composition according to claim 1 or claim 2 in which the sulphide cure control agent (D) is monosulphide. A composition according to claim 1 in which the sulphide cure control agent (D) is selected from S(-C 12 H 23 ) 2 , S 2 (-C 10 H 19 ) 2 , S(-CH 2 -Ph) 2 , H 11 C 5 - S - C 12 H 23 , Ph - C 2 H 4 - S - S - C 10 H 21 , A composition according to any one of the preceding claims in which the sulphide cure control agent (D) is used at 0.01 to 1 parts by weight per 100 parts of the organopolysiloxane component (A). A composition according to any one of the preceding claims further comprising at least one selected from vinyl siloxanes of a degree of polymerisation from 3 to 20; hydroperoxides; ethylenically-unsaturated isocyanurates; acetylene alcohols, and peroxides containing carbon-carbon unsaturation. A composition according to any one of the preceding claims comprising a silica powder filler for reinforcement at from 5 to 70 parts by weight to 100 parts of the organopolysiloxane (A). A composition according to any one of the preceding claims comprising a blowing agent for sponge formation. A composition according to any one of the preceding claims in which the mentioned ingredients are provided mixed as a one-pack composition for storage. The use of sulphide according with the general formula R 6 -(S) m -R 7 wherein each of R 6 and R 7 is monovalent hydrocarbon having at least 3 carbon atoms, or monovalent hydrocarbon including an ester group, or R 6 and R 7 together form a ring structure, as curing control agent to inhibit room-temperature gelation in relation to the addition-curing reaction of a one-pack silicone rubber composition comprising (A) organopolysiloxane of the following average compositional formula (1): R 1 n SiO (4 - n)/2 wherein groups R 1 , which may be identical or different, are selected from substituted and unsubstituted monovalent hydrocarbon groups and include at least two aliphatic unsaturated groups per molecule, and n is in the range 1.98 to 2.02; (B) organohydrogenpolysiloxane of the following average compositional formula (2) : R 2 a H b SiO (4 - a - b)/2 having per molecule at least two hydrogen atoms attached to silicon atom, and wherein groups R 2 , which may be identical or different, are selected from substituted and unsubstituted monovalent hydrocarbon groups and a and b satisfy the relationships 0 ≤ a < 3, 0.005 ≤ b ≤ 2 and 0.8 ≤ a + b ≤ 3.0, and (C) a platinum series catalyst.

Inventors: HIRABAYASHI SATAO, NAKAMURA TSUTOMU, HIRABAYASHI, SATAO, NAKAMURA, TSUTOMU, HIRABAYASHI, SATAO, C/O SILICONE-ELEC. MATERIALS, NAKAMURA, TSUTOMU, C/O SILICONE-ELEC. MATERIALS
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1994
Main classification: C08K5
IPC codes: C08K5